1. Field of the Invention
The present invention relates generally to a hydraulic shock absorber, suitable for use in an automotive suspension system. More specifically, the invention relates to a valve construction in a hydraulic shock absorber.
2. Field of the Invention
Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 62-107131 discloses one of typical construction of a conventional shock absorber. The disclosed shock absorber has a piston body separating the internal space of a shock absorber cylinder into upper and lower fluid chambers, and a base body which separates the lower fluid chamber from a reservoir chamber defined between inner and outer cylinders. The piston body and the base body are formed with orifices. An annular projection with plane tip end surface is formed at an orientation radially outward of the orifices. A constant orifice is formed through the annular projection. A valve plate is biased and seated onto the plane tip end surface.
Accordingly, in the relatively low piston stroke, valve plate is maintained in contact with the contacting surface of the annular projection. Therefore, the working fluid only through the constant orifice to generate absorption in a characteristics proportional to square of the piston stroke speed. On the other hand, in the intermediate and high piston stroke speed, the valve plate is deformed to generate absorption in a characteristics proportional to two over three power of the piston stroke speed. With such construction, when the piston stroke speed is lower then a relief speed at which the valve plate is deformed to open, all of the working fluid flows through constant orifice of constant path area. Therefore, the absorption characteristics becomes proportional to two power of the piston stroke speed. Accordingly, leakage through gaps between the piston and cylinder and between the piston rod and guide will cause substantial influence for the shock absorbing characteristics. In order words, the such leakage causes equivalent effect to variation of the path area in the constant orifice. Therefore, fluctuation of shock absorbing characteristics particularly at the low piston stroke range becomes significant.
In addition, when the absorption is set at the piston stroke speed of 0.1 m/s, the absorption characteristics can be uniformly determined in a piston stroke speed range lower than the set speed. Therefore, freedom of setting of the absorption characteristics is unacceptably narrow.